[thehappypm]

Might even be doable without water vapor at cold enough temperatures. Vapor could damage or cause the rice/beans to start to rot later on, dead giveaway of tampering. Maybe the plastic beads are better in general because of their lack of moisture and general lack of organic weirdness.

[pvillano]

plastic beads are also infinitely reusable, although probably have a much higher environmental impact than grains. Gravel could work.